1. Field of the Invention
This invention relates to passively cooled repeater housings for use in a telecommunication network's wire transmission local loop outside plant and more specifically to repeater housings having improved thermal transfer characteristics, improved performance under solar loads and direct access to repeaters and voltage surge protectors.
2. Description of the Related Art
In the telecommunications industry, voice, data and video transmission signals are transmitted over wire, fiber optic and wireless networks. Although the fiber optic and wireless networks were designed to meet the current demand for high speed signal transmission, the massive investment in the wire network, or the "copper plant" as it is commonly referred to, necessitates its continued use. The cost and time involved to completely replace the millions, if not billions, of miles of copper (or aluminum) wires in the United States alone with fiber optic lines and wireless networks is prohibitive. Although originally designed to carry only voice grade signals, the continued development of increasingly sophisticated digital signal processing (DSP) techniques such as T-carrier, ISDN, Direct Digital Service (DDS) and most recently, High bit-rate Digital Subscriber Line (HDSL) allow the telecommunications industry to transmit rapidly growing volumes of high speed signals over the copper plant in a more cost effective manner than conversion to the newer transmission technologies in all but the high volume networks.
As shown in FIG. 1, a typical telecom network 10 includes a number of central offices 12 that transmit a massive amount of very high speed signals between offices over inter-office trunks 14 and a number of local loops 16 that distribute portions of the signals from a central office 12 to a customer premises 18 and between customer premises 18. A clear distinction has existed between the offices' inter-office trunks 14 and the local loops 16. First, each central office will typically service many user premises. As a result, the cost of replacing the copper plant for the central offices' inter-office trunks is much lower than replacing it for all the individual users. Second, the signal traffic between central offices is typically much higher volume and much higher speed than is required in the local loop.
As a result, inter-office trunks 14 have been largely converted from copper wire to the more sophisticated fiber optics, microwave transmission and even satellite transmission systems while the local loops have used the updated DSP technologies in conjunction with the existing and even new installation copper 20. However, for the last several years, the explosive growth in demand for high speed telecommunications services such as those required for business networking and the Internet has been stressing the capabilities of the copper network in the local loop.
One particular area in which the copper network is being stressed occurs in the "outside plant", i.e. that part of the local loop that lies outside the controlled environments of telecom or user buildings, generally regarded as the lowest technology link in the network and symbolized by the lineman on a pole or in a manhole.
As signals are transmitted over the copper wires in the outside plant they degrade and lose signal integrity. As a result, the industry has developed circuits called "mid-span repeaters" or simply "repeaters" that regenerate a degraded signal. Depending upon the transmission technology used, the repeaters are placed every three to twelve thousand feet along the transmission path.
Repeaters are manufactured by numerous suppliers to support a variety of copper transmission technologies. Several industry standard connector and case standards are followed to allow repeaters from different suppliers and of different technologies to be interchanged. FIGS. 2a and 2b illustrate a standard 239 mini-repeater 22 often used with older T-1 technology and a standard 239 double-wide repeater 24 that is commonly used with the ISDN, DDS and HDSL technologies. The T-1 239 mini-repeater generates approximately 0.75 watts of waste heat whereas an HDSL 239 double-wide, while only twice as big, generates up to 6 watts of waste heat. Because of the nearly order of magnitude increase in power consumption, the 239 double-wide is frequently provided with slits that facilitate air flow over the hot parts to convectively remove heat. The power consumption of ISDN and DDS repeaters is also substantially greater than T-1 239 mini-repeaters, but less than that of the more sophisticated HDSL repeaters.
Because mid-span repeaters are used in the outside plant, frequently in manholes 28, they must be placed in a repeater housing 26 such as the AT&T '809 Apparatus Case 30 shown in FIGS. 3a and 3b, the SPC Series 7000 Enclosure 32 shown in FIGS. 3c and 3d, or the generic cabinet style enclosure 34 shown in FIG. 3e. The primary function of these known repeater housings is to provide an environmental enclosure that shields the repeaters from the elements; wind, rain, dust, solar energy, animals, vandals etc. They are oftentimes formed from strong corrosion resistant materials such as stainless steel or hard plastic and are hermetically sealed, often under a positive pressure. In mild environments the repeater housing do not have be corrosion resistant and above ground cases are often vented.
The housings must accommodate standard sized repeater modules that are built by a number of vendors. The housings must also provide physical access to repeater modules and voltage surge protectors so that they can be removed and replaced in the field in a "plug-in" manner without having to disassemble the module or disturb the operation of other repeaters. Furthermore, to improve reliability and avoid the expense of requiring electrical power at each repeater site, the housing must be passively cooled to remove the waste heat generated by the repeaters. It is well understood in the telecommunications industry that thermal stress can cause short term failures, intermittent operation deviations and significantly shorten the life of electronic equipment. Most telecom electronics is installed in buildings that provide a controlled and relatively benign thermal environment. In contrast, repeaters deployed in the outside plant must work in the harsh, natural environment.
The AT&T '809 apparatus case 30 shown in detail in FIG. 4 and the double size '819 apparatus case described in AT&T Practice 640-525-307 Issue 5, April 1986 is a molded plastic rectangular housing that is lightweight, does not corrode, and optimizes the use of available space. The '819 obsoleted AT&T's earlier '479 apparatus case described in AT&T Practice 640-527-107 Issue 3, March 1986 that had the same general shape but was constructed from cast iron, and thus extremely heavy and subject to corrosion.
The '809 includes a molded base 36 for receiving a stub cable 38 from a splice case in the local loop and a mounting bracket 40 for mount the case on the wall of a manhole, for example. Pressure and pressure relief valves are also provided in the base. Stub cable 38 is split into individual wires that are run through base 36 and wire-wrapped to the backside of repeater/protector connectors 42, which have a female PCB edge connector 44 for mounting the repeater module and multiple sockets 46 for mounting gas tube style voltage surge protectors 48.
A molded housing 50 having an array of plastic stubs 52 is bolted on base 36 so that stubs 52 define slots 54 over the respective repeater/protector connectors 42 for separating and supporting the repeater modules. A molded cover 56 is then bolted on top of housing 50. The cover can be removed to gain direct access to the top of the enclosed repeater modules for easy installation and replacement. The illustrated '809 case is designed, physically and electrically, to hold 12 239 mini-repeater modules 22 or 6 non-standard repeater modules 25 with 2 slots used for support functions. The '809 case was designed for the 239 mini-repeater and thus does not physically accommodate the standard 239 double-wide case. Some suppliers have developed a variation of the 239 double-wide that is even wider and has slots 58 in the case to allow it to fit into two slots in the '809 and '819.
To make the best use of the space available inside housing 50, voltage surge protectors 48 are positioned in connector sockets underneath the repeater modules. To gain access to the protectors, a lineman must first remove the repeater module, taking it out of service temporarily. Because the voltage surge protectors are positioned at the bottom of narrow slots 54 they can be very difficult to remove. Consequently, AT&T provides a special 829A tool and a detailed multi-step process for extracting the gas protectors. In practice, lineman sometimes use a long screwdriver to pop the protectors loose. However, with +/-130 volts active on the contacts of the protector sockets, attempted service without the proper tool can be hazardous, both to the lineman and to the equipment.
Although the practice makes no mention of thermal considerations, the '809 relies on convection and, to a lesser degree, radiation to remove waste heat from the repeater modules. The repeater module and, in the case of the slit, modified double-wide, the components themselves heat the air which transfers some heat to the adjacent walls of the case and rises to convectively transfer the rest of the heat to the top of the case. The walls and end of the case absorb the waste heat and then convectively transfer it to the surrounding environment. Notice, stubs 52 position, but do not tightly enclose the repeater modules to encourage air flow to improve convective heat transfer to the top of the case.
The SPC 7000 Series enclosure 32 shown in FIGS. 5 and 6 is a thin-walled stainless steel cylindrical enclosure. The 7000 Series enclosure includes a cylindrical base 60 into which it receives a stub cable 62. A lightweight thin aluminum basket 64 is centrally mounted on a bracket 66 in base 60. A number of female PCB repeater connectors 68 are mounted in slots in the bottom of the basket with their pins 70 wire wrapped (not shown) to the stub cable. A voltage surge protector assembly 72 is then mounted on the back side of connector 68 and repeater modules 24 are mounted on the top side in the basket. Bracket 66 allows basket 64 to be tipped to access the backside wiring. Alternately, the basket can be replaced with a chassis in which the modules are inserted horizontally from one side and access to the protectors is gained from the other side. A dome 74 fits over base 60 and is hermetically sealed using a V-groove clamp 76, an O-ring 78a and an O-ring retainer 78b. Similar to the '819, the Series 7000 enclosure relies on radiation and convection to move the waste heat generated by the repeater modules to the dome and then to the surrounding environment. To this end, the 239 double-wide repeater modules and the basket are formed with slits to encourage air flow. In its normal upright position, the heated air rises to the top of the dome where it is then convectively transferred to the environment.
Access to the repeater is gained by removing the entire cylindrical dome. The removal of the entire cylindrical dome of an SPC 7000 style repeater housing is of little consequence in above ground and low density below ground (manhole) installations, however, with sharply increased crowding in below ground facilities, the extra clearance required to remove the entire dome has become a drawback to the otherwise satisfactory cylindrical dome repeater housing configuration. In response, repeater housing mounting brackets have been developed that allow the entire housing to pivot away from the mounting surface sufficiently to permit the cylindrical dome to be removed without requiring excessive vertical clearance directly above its installation.
While such access to the modules and voltage protectors may seem convenient, in the reality of a crowded manhole, "tilt, swivel and around the back" represent difficult access, excessive installation and service time and risk to the hundreds of wires connected to the repeater modules. Furthermore, the +/-130 VDC span power is not deactivated during service of the repeater housing, therefore, installation and removal of the voltage protector assemblies connected to this voltage must be performed with serious caution.
The '819 apparatus case and Series 7000 enclosure, and the thermal transfer techniques they embody, are designed to handle up to 25 239 mini-repeaters and do so with no problem. However, those same housings are limited to 2 or maybe 3 HDSL repeaters before their thermal transfer capabilities are overloaded. With the demand for high bandwidth service continuing to grow and the amount of available space either below ground in manholes or above ground reaching or exceeding capacity, repeater housings in which a majority of the slots must remain empty for thermal reasons is clearly a problem. Furthermore, direct, easy and safe access to the repeater modules and their voltage surge protectors is an important consideration.